The present disclosure relates to a recording apparatus configured to illuminate an optical disc recording medium having a reference surface on which address information is recorded by formation of a position guiding element and a recording layer which is formed at a different depth position from that of the reference surface, with recording light for performing mark recording on the recording layer and light for position control for performing position control based on the position guiding element formed on the reference surface, via a common objective lens, and a method thereof.
As optical disc recording media (optical discs) for recording and reproducing signals by light illumination, for example, CDs (Compact Discs), DVDs (Digital Versatile Discs), and BDs (Blu-ray Discs: registered trademark) have been popularized.
With regard to the next-generation optical discs of the currently popularized optical discs such as CDs, DVDs, and BDs, the applicant has previously proposed a so-called bulk recording-type optical disc as disclosed in Japanese Unexamined Patent Application Publication No. 2008-135144 and Japanese Unexamined Patent Application Publication No. 2008-176902.
Here, bulk recording is, for example, a technique for performing multi-layer recording on a bulk layer 102 by performing laser light illumination while sequentially changing focal positions in an optical recording medium (a bulk-type recording medium 100) having at least a cover layer 101 and the bulk layer (recording layer) 102 as illustrated in FIG. 19, thereby achieving an increase in recording capacity.
For the bulk recording, in Japanese Unexamined Patent Application Publication No. 2008-135144, a recording technique called a microhologram method is disclosed.
In the microhologram method, as a recording material of the bulk layer 102, a so-called hologram recording material is used. As the hologram recording material, for example, photopolymerizable polymer or the like is widely used.
The microhologram method is mainly classified into a positive-type microhologram method and a negative-type microhologram method.
The positive-type microhologram method is a technique for condensing two opposing light beams (a light beam A and a light beam B) at the same position to form a fine fringe (hologram) which becomes a recording mark.
In addition, the negative-type microhologram method is, using the opposite idea to the positive-type microhologram method, a technique for erasing a fringe which is formed in advance through laser light illumination to use the erasure portion as a recording mark. Specifically, in the negative-type microhologram method, before performing a recording operation, an initialization process for forming a fringe on the bulk layer 102 is performed in advance. That is, light beams C and D by parallel light are emitted to be opposed so as to form the fringes on the entirety of the bulk layer 102. In addition, after the fringe is formed in advance by the initialization process, information recording is performed by forming erasure marks. Specifically, by performing laser light illumination according to the information to be recorded in a state where laser beams are focused at an arbitrary layer position, the information recording using erasure marks is performed.
In addition, the applicant also proposes, as a bulk recording technique different from the microhologram method, a recording technique for forming voids (holes and blanks) as recording marks, for example, as disclosed in Japanese Unexamined Patent Application Publication No. 2008-176902.
The void recording method is a technique for performing laser light illumination on the bulk layer 102 made of a recording material such as photopolymerizable polymer at a relatively high power, thereby recording holes in the bulk layer 102. As disclosed in Japanese Unexamined Patent Application Publication No. 2008-176902, the hole portions formed as described above become portions having different refractive indexes from other portions in the bulk layer 102, and reflectance of light at the boundaries thereof can be enhanced. Therefore, the hole portions function as recording marks, and accordingly information recording using the formation of hole marks is realized.
In such void recording methods, since holograms are not formed, recording is done when light illumination is performed from one side. That is, unlike the positive-type microhologram method, two light beams may not be condensed at the same position to form recording marks.
In addition, in comparison to the negative-type microhologram method, there is an advantage in that an initialization process may not be performed.
In addition, in Japanese Unexamined Patent Application Publication No. 2008-176902, an example in which illumination of the light for pre-curing is performed before recording when void recording is to be performed is described. However, recording of the voids can be made even when the illumination of the light for pre-curing is omitted.
However, although various recording techniques as described above have been proposed for bulk recording-type (simply referred to as bulk-type) optical recording media, a recording layer (bulk layer) of such a bulk-type optical recording medium does not have an explicit multi-layer structure in the sense that, for example, a plurality of reflection films are formed. That is, the bulk layer 102 is not provided with a reflection film and a guiding groove that may be provided in a typical multi-layer disc for each recording layer.
Therefore, in the structure of the bulk-type recording medium 100 illustrated in FIG. 19 as it is, during recording without marks being formed, focus servo or tracking servo may not be performed.
Accordingly, in practice, the bulk-type recording medium 100 is provided with a reflection surface (reference surface Ref) which is the reference having a guiding groove as illustrated in FIG. 20.
Specifically, guiding grooves (position guiding elements) such as pits or grooves are formed on the lower surface side of the cover layer 101, and a selective reflection film 103 is formed thereon. In addition, on the lower layer side of the cover layer 101 on which the selective reflection film 103 is formed, the bulk layer 102 is laminated via an adhesive material such as UV-curable resin as an intermediate layer 104 in FIG. 20.
In addition, in this medium structure, the bulk-type recording medium 100 is, as illustrated in FIG. 21, illuminated with, separately from a laser light for recording marks (a laser light for recording), a laser light for servo as a laser light for position control.
As illustrated in FIG. 21, the laser light for recording and the laser light for servo illuminate the bulk-type recording medium 100 via a common objective lens.
Here, if the laser light for servo reaches the bulk layer 102, there is a concern that the laser light for servo may have an adverse effect on mark recording on the bulk layer 102. Accordingly, in a bulk recording method according to the related art, a laser light having a wavelength band different from that of the laser light for recording is used as the laser light for servo, and the selective reflection film 103 which has wavelength selectivity in that it reflects the laser light for servo and transmits the laser light for recording is provided as a reflection film formed on a guiding groove formation surface (reference surface Ref).
On the above-described premise, operations performed during mark recording on the bulk-type recording medium 100 will be described with reference to FIG. 21.
First, when multi-layer recording is to be performed on the bulk layer 102 without a guiding groove or a reflection film being formed, a layer position at which marks are recorded in a depth direction on the bulk layer 102 is set in advance. In FIG. 21, a case is exemplified where as layer positions at which the marks are to be formed (mark formation layer positions: also called information recording layer positions) in the bulk layer 102, first to fifth information recording layer positions L1 to L5, making a total of 5 information recording layer positions L, are set. As illustrated, the first information recording layer position L1 is set to a position at a first offset of of-L1 in a focus direction (depth direction) from the selective reflection film 103 (the reference surface Ref) provided with guiding grooves. In addition, the second, third, fourth, and fifth information recording layer positions L2, L3, L4, and L5 are respectively set to positions at second, third, fourth, and fifth offsets of of-L2, of-L3, of-L4, and of-L5 from the reference surface Ref.
During recording in which marks are not formed yet, focus servo or tracking servo may not be performed on each of the layer positions L as an object in the bulk layer 102 on the basis of reflected light of the laser light for recording. Therefore, during recording, focus servo control and tracking servo control of the objective lens are performed on the basis of the reflected light of the laser light for servo as the light for position control so that a spot position of the laser light for servo follows the guiding grooves on the reference surface Ref.
However, the laser light for recording has to reach the bulk layer 102 formed on a lower layer side in relation to the selective reflection film 103 for mark recording. Accordingly, in this optical system, separately from a focus mechanism of the objective lens, a focus mechanism is provided for independently adjusting a focal position of the laser light for recording.
Here, an internal configuration example of the recording apparatus of the bulk-type recording medium 100 including the mechanism for independently adjusting the focal position of the laser light for recording is illustrated in FIG. 22.
In FIG. 22, a first laser diode 111 denoted by LD1 in FIG. 22 is a light source of the laser light for recording, and a second laser diode 119 denoted by LD2 is a light source of the laser light for servo. As understood from the above description, the first and second laser diodes 111 and 119 are adopted to emit laser lights having different wavelength bands from each other.
As illustrated in FIG. 22, the laser light for recording emitted by the first laser diode 111 is incident on the focus mechanism constituted by a fixed lens 113, a movable lens 114, and a lens driving unit 115 via a collimation lens 112. As the movable lens 114 is driven by the lens driving unit 115 in a direction parallel to an optical axis of the laser light for recording, the collimation state (convergent, parallel, or divergent state) of the laser light for recording incident on an objective lens 117 in FIG. 22 is changed, so that the focal position of the laser light for recording can be adjusted independently from a change in focal position that is caused by driving the objective lens 117.
In this sense, the focus mechanism is also referred to as a focus mechanism for a recording light.
The laser light for recording transmitted via the focus mechanism for a recording light is incident on a dichroic mirror (dichroic prism) 116 adopted to transmit light having the same wavelength band as that of the laser light for recording and reflect light having different wavelength bands.
As illustrated, the laser light for recording transmitting the dichroic mirror 116 illuminates the bulk-type recording medium 100 via the objective lens 117. The objective lens 117 is held to be displaced in a focus direction and a tracking direction by a biaxial actuator 118.
In addition, the laser light for servo emitted by the second laser diode 119 is transmitted through a beam splitter 121 via a collimation lens 120 and is incident on the above-mentioned dichroic mirror 116. The laser light for servo reflects from the dichroic mirror 116 and is incident on the objective lens 117 so that its optical axis is aligned with the optical axis of the laser light for recording transmitting the dichroic mirror 116.
The laser light for servo incident on the objective lens 117 is focused on the selective reflection film 103 (the reference surface Ref) of the bulk-type recording medium 100 as the biaxial actuator 118 is driven under focus servo control by a servo circuit 125 described later. Simultaneously, the position of the laser light for servo in the tracking direction is caused to follow the guiding groove formed on the selective reflection film 103 as the biaxial actuator 118 is driven under tracking servo control by the servo circuit 125.
The reflected light of the laser light for servo reflected from the selective reflection film 103 is reflected from the dichroic mirror 116 via the objective lens 117 and is then reflected again from the beam splitter 121. The reflected light of the laser light for servo reflected from the beam splitter 121 is condensed on a detection surface of a photodetector 123 via a condenser lens 122.
A matrix circuit 124 generates focusing and tracking error signals on the basis of light sensing signals detected by the photodetector 123 and supplies the error signals to the servo circuit 125.
The servo circuit 125 generates a focus servo signal and a tracking servo signal from the error signals. As the above-mentioned biaxial actuator 118 is driven on the basis of the focus servo signal and the tracking error signal, the focus servo control and the tracking servo control of the objective lens 117 are realized.
Here, when mark recording is to be performed on a given information recording layer position L as an object selected from among the information recording layers position L set in advance in the bulk-type recording medium 100, the operation of the lens driving unit 115 is controlled to change the focal position of the laser light for recording by the offset of corresponding to the selected information recording layer position L.
Specifically, setting control of such an information recording position is performed by, for example, a controller 126 that controls the entire recording apparatus. That is, the operation of the lens driving unit 115 is controlled by the controller 126 on the basis of an offset amount of-L set in advance according to the information recording layer Ln as the object, thereby setting the information recording position (focal position) of the laser light for recording to the information recording layer position Ln which is the object.
In addition, during recording, the tracking servo of the laser light for recording is automatically performed as the tracking servo control of the objective lens 117 is performed by the servo circuit 125 on the basis of the reflected light of the laser light for servo as described above. Specifically, the spot position of the laser light for recording in the tracking direction is controlled to be immediately under the guiding groove formed on the reference surface Ref.
Moreover, when the bulk-type recording medium 100 on which the mark recording is already performed is reproduced, the position of the objective lens 117 may not be controlled on the basis of the reflected light of the laser light for servo from a reference surface Ref unlike during recording. That is, during reproduction, mark rows as objects formed on the information recording layer position L as a reproduction object are illuminated with a laser light for reproduction, thereby performing the focus servo control and the tracking servo control of the objective lens 117 on the basis of the reflected light of the laser light for reproduction.
As described above, in the bulk recording method, the bulk-type recording medium 100 is illuminated with the laser light for recording as the mark recording light and the laser light for servo as the light for position control via the common objective lens 117 (to be combined on the same optical axis). Thereafter, the focus servo control and the tracking servo control of the objective lens 117 are performed on the basis of the reflected light of the laser light for servo, so that the focus servo and the tracking servo of the laser light for recording can be performed even though the guiding grooves or a reflection surface having the guiding groove formed therein are not formed on the bulk layer 102.
However, when the servo control technique as described above is employed, there is a problem in that due to a lens shift of the objective lens 117 caused by the eccentricity of the bulk-type recording medium 100, a deviation in the information recording position in the tracking direction occurs.
FIGS. 23A to 23C are diagrams illustrating principles of generating deviations of the information recording position caused by the lens shift as described above.
In FIGS. 23A to 23C, FIG. 23A illustrates an ideal state in which there is no eccentricity of the bulk-type recording medium 100 and a lens shift of the objective lens 117 does not occur. FIG. 23B illustrates a case where a lens shift (referred to as (+) direction eccentricity) occurs in the left direction of the figure (referred to as the outer peripheral direction). FIG. 23C illustrates a case where a lens shift (referred to as (−) direction eccentricity) occurs in the right direction of the figure (referred to as the inner peripheral direction).
First, the center axis c in FIGS. 23A to 23C is a center axis set to design an optical system, and in the ideal state illustrated in FIG. 23A, the center of the objective lens 117 is aligned with the center axis c.
Contrary to this, when the lens shift in the (+) direction occurs as illustrated in FIG. 23B, the center of the objective lens 117 is shifted to the (+) direction with respect to the center axis c of the optical system.
Here, since the laser light for servo (a patterned light beam in FIGS. 23A to 23C) is incident on the objective lens 117 as a parallel light, even though there is a shift of the objective lens 117 from the center axis c as described above, a change in the focal position of the laser light for servo in the tracking direction does not occur. Contrary to this, since the laser light for recording (an outlined light beam in FIGS. 23A to 23C) is incident on the objective lens 117 so as not to be parallel therewith so as to be focused on the selected information recording layer position L in the bulk layer 102 on the lower layer side than the reference surface Ref as described above, by the shift of the objective lens 117 in the (+) direction as described above, as in FIG. 23B, the focal position (information recording position) of the laser light for recording is changed in the (+) direction by the lens shift amount (a deviation amount +d in FIG. 23B).
In addition, when a lens shift in the (−) direction as illustrated in FIG. 23C occurs, the information recording position of the laser light for recording is changed in the (−) direction by the lens shift amount as illustrated in FIG. 23C (a deviation amount −d in FIG. 23C).
As such, the configuration of the recording apparatus for the bulk-type recording medium 100 described above with reference to FIG. 22 is implemented so that:                the laser light for recording and the laser light for servo are illuminated via the common objective lens 117,        the focus servo control of the objective lens 117 is performed to focus the laser light for servo on the reference surface Ref of the bulk-type recording medium 100,        the focal position (the information recording position) of the laser light for recording is adjusted by changing the collimation state of the laser light for recording incident on the objective lens 117, and        the tracking servo control of the objective lens 117 is performed to cause the focal position of the laser light for servo to follow the guiding groove formed on the reference surface Ref.        
In this configuration, there is a problem in that the information recording position of the laser light for recording is deviated in the tracking direction due to the eccentricity of the disc.
Here, depending on a degree of the eccentricity or setting of a track pitch (an interval between the guiding grooves formed therein), there may be a case where the information recording positions of adjacent guiding grooves may be overlapped. In this case, a recording signal is not correctly reproduced.
In addition, the lens shift of the objective lens 117 has been described as a main factor of the deviation of the information recording position; however, the deviation of the information recording position is also caused by a disc tilt.